Math and Number Theory coding problems

Math problems in coding rounds: primes, GCD and LCM, digit tricks, modular arithmetic and counting. Short to write, easy to get wrong on edge cases.

36 problems: 17 easy, 14 medium, 5 hard. Free to read; a free account solves them and earns XP and Aura.

1. Count DigitsPractice · +10 XP · recursion, matheasy
2. Count DivisorsPractice · +10 XP · basics, loops, arithmeticeasy
3. Count Elements Greater Than AveragePractice · +10 XP · basics, loops, arithmeticeasy
4. Count MultiplesPractice · +10 XP · binary search, matheasy
5. Count Multiples of KPractice · +10 XP · dynamic programming, iteration, number theoryeasy
6. Count Perfect SquaresPractice · +10 XP · math, loopseasy
7. Number of DivisorsPractice · +10 XP · math, number theoryeasy
8. Print Digits of a NumberPractice · +10 XP · basics, loops, arithmeticeasy
9. Print Nth Fibonacci NumberPractice · +10 XP · basics, loops, arithmeticeasy
10. Recursive FactorialPractice · +10 XP · recursion, matheasy
11. Reverse DigitsPractice · +10 XP · math, digitseasy
12. Sum of DigitsPractice · +10 XP · recursion, arithmeticeasy
13. Sum of DivisorsPractice · +10 XP · math, number theoryeasy
14. Sum of Even NumbersPractice · +10 XP · basics, loops, arithmeticeasy
15. Sum of Multiples of Three and FivePractice · +10 XP · math, loops, divisibilityeasy
16. Sum of Odd DigitsPractice · +10 XP · math, digits, loopseasy
17. Sum of Proper DivisorsPractice · +10 XP · math, number theoryeasy
18. Count Distinct Prefix SumsPractice · +20 XP · math, prefix sums, hashingmedium
19. Count Pairs with Digit Sum Divisible by KPractice · +20 XP · math, hashing, prefix sumsmedium
20. Count Pairs with Equal Digit GCDPractice · +20 XP · math, number theory, arraysmedium
21. Count Pairs with Equal Digit ProductPractice · +20 XP · math, hashing, number theorymedium
22. Count Pairs with Equal Digit SumPractice · +20 XP · math, hashing, arraysmedium
23. Count Pairs with Sum Divisible by KPractice · +20 XP · arrays, hashing, number theorymedium
24. Distinct Prime FactorsPractice · +20 XP · math, number theory, primesmedium
25. Divisible by Last DigitPractice · +20 XP · math, number theory, hash mapmedium
26. Pairs with Common FactorPractice · +20 XP · math, number theory, prime factorizationmedium
27. Pairs with Same ParityPractice · +20 XP · sorting, two pointers, combinatoricsmedium
28. Pairs with Sum Divisible by KPractice · +20 XP · math, number theory, hash mapmedium
29. Subarray Digit SumsPractice · +20 XP · math, prefix sums, arraysmedium
30. Subarray Sum MultiplesPractice · +20 XP · prefix sums, hash maps, number theorymedium
31. Two Sum Modulo KPractice · +20 XP · math, hashing, number theorymedium
32. Array Transformation with GCDPractice · +30 XP · math, number theory, gcdhard
33. Distinct Subarrays with Bounded SumPractice · +30 XP · two pointers, hash map, combinatoricshard
34. GCD of SubarraysPractice · +30 XP · math, number theory, gcdhard
35. Prefix Sums and ParityPractice · +30 XP · prefix sums, parity, combinatoricshard
36. The Harmonious SequenceOpen now · Monthly · +150 XP · +50 Aura · math, number theory, dynamic programming2 solvedhard